Timepiece display devices that match the above definition are already known. CH Patent No. 693,155 discloses a switch mechanism that includes two heart-pieces carried by two wheel sets whose angular positions are respectively representative of two variable magnitudes. The first wheel set and the heart-piece that it carries are driven in rotation by the timepiece movement. Moreover, in a similar manner to a device that is known in split-seconds chronographs, a wheel carrying a hand is freely mounted on the rotational arbour of the first wheel set. This wheel carries a lever, which is permanently returned against the flank of the heart-piece by a small spring.
With this mechanism of the prior art, the hand indicates the variable magnitude represented by the angular position of the second wheel set. If the person wearing the timepiece wishes to switch the display to see the first variable magnitude, he activates a switch mechanism that acts, first of all, by uncoupling the hand from the second heart-piece. Once uncoupling is carried out, the hand is then only connected to the wheel that carries the lever returned against the flank of the first heart-piece. In this case, the weak pressure exerted by the lever is enough to cause it to slide against the heart-piece flank, and thus to rotate the lever about the heart-piece with the wheel that carries it. At the end of its travel around the heart-piece, the inclined portion of the lever is locked against the base of the heart-piece, and the heart-piece is thus coupled with the hand. It will be clear that the effect of the action of the lever on the heart-piece is to bring and then keep the hand in the position that represents the state of the first variable magnitude.
According to this prior art document, switching the display in the opposite direction is considerably more complex. Indeed, for the display to pass from the first to the second variable magnitude involves a differential gear including a planetary wheel holder, which is secured to the second heart-piece of the mechanism.